postgres的強制類型轉換與時間函數

一。類型轉換
postgres的類型轉換：通常::用來做類型轉換，timestamp到date用的比較多
select  now()::date
select  now()::varchar

示例1：日期的varchar計算成date
select '2012-11-15 16:15:56.377000+08'::timestamp::date
select '2012-11-15 16:15:56.377000+08'::date
結果: 2012-11-15



二。時間的類型轉換與相對時間

//注意java的timestamp將來在sql中體現的varchar的形式‘2012-11-15 16:15:56.377000+08’，這樣的串可以計算時間差。

假如表中的一條記錄的publishdate是 '2012-11-15 16:15:56.377000+08'，想確認該記錄是不是過去24小時之內publish的記錄，可以使用如下的判斷：

select  extract(epoch from now() - '2012-11-15 16:15:56.377000+08')< 24*3600 

select now() - '2012-11-15 16:15:56.377000+08' < '24 hours'

select now() - '2012-11-15 16:15:56.377000+08' < '1 days' or select now() - '2012-11-16 16:15:56.377000+08' < '1 day'

select now()::date-'2012-11-15 16:15:56.377000+08'::date < 1


注：相對時間表示時間范圍，通常用於統計，定時任務 。除了相對時間，‘today’使用的也比較多。比如取當天的記錄使用:publishdate::date = 'today'


三。時間函數Extract用於提取絕對時間的年，月，日.....; 相對時間的秒值。

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13');
Result: 20
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 21

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 to 1. PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

day

The day (of the month) field (1--31)

SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 16

decade

The year field divided by 10

SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 200

dow

The day of the week (0--6; Sunday is 0) (for timestampvalues only)

SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 5

Note that extract's day of the week numbering is different from that of the to_char function.

doy

The day of the year (1--365/366) (for timestampvalues only)

SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 47

epoch

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for intervalvalues, the total number of seconds in the interval

SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE
 '2001-02-16 20:38:40-08');
Result: 982384720

SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
Result: 442800

Here is how you can convert an epoch value back to a time stamp:

SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720 *
 INTERVAL '1 second';

hour

The hour field (0--23)

SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 20

microseconds

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
Result: 28500000

millennium

The millennium

SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16
 20:38:40');
Result: 3

Years in the 1900s are in the second millennium. The third millennium starts January 1, 2001. PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

milliseconds

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
Result: 28500

minute

The minutes field (0--59)

SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 38

month

For timestamp values, the number of the month within the year (1--12) ; for intervalvalues the number of months, modulo 12 (0--11)

SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 2

SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');
Result: 3

SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months');
Result: 1

quarter

The quarter of the year (1--4) that the day is in (for timestampvalues only)

SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 1

second

The seconds field, including fractional parts (0 - 59 ⁽³⁾)

SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 40

SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
Result: 28.5

timezone

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

timezone_hour

The hour component of the time zone offset

timezone_minute

The minute component of the time zone offset

week

The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year. (for timestamp values only) Because of this, it is possible for early January dates to be part of the 52nd or 53rd week of the previous year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01is part of the 52nd week of year 2005.

SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 7

year

The year field. Keep in mind there is no 0 AD, so subtracting BC years from ADyears should be done with care.

SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 2001

The extract function is primarily intended for computational processing. For formatting date/time values for display, see section 7.8 Data Type Formatting Functions.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

date_part('field', source)

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
Result: 16

SELECT date_part('hour', INTERVAL '4 hours 3 minutes');
Result: 4